The purpose of this study was to determine the biliary metabolites of tirilizad mesylate in human subjects. Volunteers had bile collected by nasobiliary catheters following intravenous administration of tirilizad. The bile was collected, frozen and dispatched to UpJohn Company's laboratories for analysis. Using a novel mass spectrometry assay (CRIMS technique), stable isotope labeled metabolites of tirilizad were detected, allowing the unique metabolic pathway of this drug in humans to be determined. This study proves that the CRIMS technique can detect minute quantities of drug metabolites in human bile recovered by nasobiliary drainage.